Add RepeatN operator adapter

[dbalcomb]

This adds a new RepeatN operator adapter.

The Repeat operator adapter uses a parameter to select how many times the interior operator will be repeated. This works well for the Mutator, Recombinator and Evolver operators but the Selector implementation returns a Vec because it cannot know how many individuals will be produced at compile time. This requires users to then use the Take selector adapter to specify a const generic number in order to use the recombinators. The project should provide an alternative repeat operator that uses a constant generic instead.

This change introduces the RepeatN operator adapter that works much the same way as the Repeat operator adapter with the exception that it uses a const generic number instead of a parameter. The implementation of the Selector operator requires that the base selector return a single individual rather than multiple. This then allows it to return N individuals based on the const generic.